1. Field of the Invention
The present invention relates to a method of producing a soft magnetic material, in particular, a ferrite thin film, for use in a transformer, an inductor, an anti-EMI component and the like.
2. Description of the Related Art
In recent years, along with an increasing adoption of multi-media due to a dramatic progress in semiconductor integrated circuit technology, mobile devices such as a cellular phone, a notebook-type personal computer and an electronic pocketbook have been astonishingly popularized. These mobile devices use a battery as a driving power source. Since the battery must drive various types of devices, a single voltage of the battery is stepped up or down by use of a DCxe2x80x94DC converter so as to be supplied to each device. On the other hand, with performance enhancement, downsizing and weight reduction of the mobile devices being advanced, downsizing and weight reduction of the DCxe2x80x94DC converter are also increasingly demanded. To meet this demand, soft magnetic parts of thin film type for a transformer and an inductor for the DCxe2x80x94DC converter are under development.
Also, it is a recent trend that the frequency used for the transmission of signals of electronic devices is made higher and higher. As a result, it can happen that the electromagnetic energy generated by a device system travels through an electrical wire as a voltage or current fluctuation or propagates in space as an electromagnetic wave, causing reception difficulty or malfunction in other device systems. For this reason, device systems must emit as limited jamming waves as possible and must also be electromagnetically tolerant so as not to be disturbed by jamming waves from other device systems. Accordingly, anti-EMI components have been increasingly employed. And the anti-EMI components are also requested to be downsized and reduced in weight, which has been driving the components into thin film type.
Conventionally, for thin film magnetic parts used for the above-mentioned transformers, inductors, anti-EMI components and so forth, Nixe2x80x94Fe based, Coxe2x80x94Znxe2x80x94Nb based metal magnetic thin films have been generally used because they can be relatively readily formed by a sputtering method. However, the metal magnetic thin films usually have to have a large inductance so they have to be formed relatively thick, for example, with a thickness of several micrometers (xcexcm). A film with such a thickness has a high electrical conductivity and is apt to suffer eddy current loss in a high frequency domain, which causes an extreme deterioration in soft magnetic properties.
On the other hand, a ferrite is known as a soft magnetic material that has a low electrical conductivity and exhibits an excellent soft magnetism even in a high frequency domain. The ferrite used as a thin film magnetic part for the above-mentioned transformer, inductor or the like will realize a small size and lightweight electronic device that can function satisfactorily in a high frequency domain.
However, there has been no firm technology established for film formation of a ferrite and hence various methods have been practiced by trial and error. The methods include a general purpose method such as a sputtering method, an evaporation method and a plating method, and also include a powder beam method in which a high flow rate gas is used as a carrier gas for spraying ferrite fine particles onto a substrate, a sol/gel method in which sol/gel is applied by spin coating or dipping, a plasma MOCVD method in which a raw material gas is converted into plasma thereby laminating a ferrite on a substrate (Japanese Patent Laid-open Nos. Hei 08-138934 and Hei 08-335514, etc.) and so forth.
In the above-mentioned sputtering method or evaporation method, the film formation rate is low. Accordingly, it takes a long time to produce a thin film with relatively large thick such as several micrometers (xcexcm), which prohibits a desired productivity. In addition, an expensive equipment is required for the method inevitably pushing up production costs.
Although the above-mentioned plating method gives a high film formation rate, the composition of the thin film or the abundance ratio between divalent iron ion (ferrous ion) and trivalent iron ion (ferric ion) changes even with a slightest fluctuation in current density. As a result, coercive field (Hc) exceeds 1 kA/m making it very difficult to obtain good soft magnetism in a stable manner.
In the above-mentioned powder beam method, a ferrite thin film with good soft magnetism can be produced at a relatively high speed. However, the thin film obtained has a rough surface and in addition a severe damage is given to the substrate.
In the above-mentioned sol/gel method, while it is difficult to obtain a film thickness of 1 xcexcm or more by spin coating, the film thickness is poorly distributed by dipping. In either case, it is difficult to obtain a thin film with a desired thickness in a stable manner. The sol/gel method requires annealing at 600xc2x0 C. or higher for crystallization, which inevitably increases the number of steps and energy consumption thereby pushing up production costs.
Furthermore, the above-mentioned plasma MOCVD method requires apparatuses such as a reduced pressure reaction chamber, a vaporizer, and a high frequency power source, incurring a high cost of equipment.
The present invention has been made in consideration of the above technical background, and an object of the present invention is to provide a method of producing a ferrite thin film that, by a simple apparatus, can efficiently form a thin film having a desired thickness without sacrificing excellent soft magnetism of ferrite thereby greatly contributing to downsizing and weight reduction of electronic devices.
To solve the above-described problems, the present invention provides a method of producing a ferrite thin film, in which a raw material mixed solution containing nitrate or alkoxide of metal that constitutes a ferrite is sprayed by means of a carrier gas onto a heated substrate.
The present invention utilizes a so-called spray pyrolysis deposition (SPD) method. In the SPD method, minute droplets containing a raw material compound (metal nitrate or metal alkoxide) on the substrate are condensed as the solvent vaporizes or gasifies, then pyrolysis deposition and chemical reaction of the raw material take place making a solid phase of the objective substance (ferrite) deposit on the substrate to form a thin film. The SPD method can be practiced in the air by a simple apparatus including an atomizer and does not require vacuum exhaust unlike the sputtering method, the evaporation method or the plasma MOCVD method. Accordingly, the expense relating to the equipment is reduced to a greater extent. In addition, the SPD method features simple operation and a low energy consumption. Also, a wide range of raw material can be selected and a ferrite film can be formed with various compositions. Moreover, a thin film with a large area can be formed.
In the present invention, the temperature of the substrate (hereinafter, sometimes referred to as xe2x80x9csubstrate temperaturexe2x80x9d) is preferably set to 300 to 500xc2x0 C. This is because the film formation process described above is critically influenced by the substrate temperature. Specifically, if the substrate temperature is too low, the above-mentioned pyrolysis deposition and chemical reaction take place insufficiently, while the solute is rapidly gasified and thermally decomposed to become powdery if the substrate temperature is too high. In either case, thin films having a good quality can be hardly formed in a stable manner.
In the present invention, the raw material mixed solution may be intermittently sprayed onto the substrate. This allows the substrate temperature to recover when the substrate mounted, for example, on a hot plate gets its temperature lowered temporarily due to the raw material mixed solution sprayed thereon. The substrate may have a heater built-in thereby constituting a self heat generating type. In this case, the substrate temperature can be kept constant by detecting the substrate temperature and feedback-controlling the heater based on the temperature detected. As a result, thin film formation by continuously spraying the raw material mixed solution is possible.
The substrate may be of a glass, ceramic, a non-magnetic metal and so forth. In the case where the substrate temperature is set to 350xc2x0 C. or less, the substrate may be of a heat-resistant resin such as polyimide.
Furthermore, the carrier gas may be compressed nitrogen gas, oxygen gas, argon gas (inert gas) and so forth as well as compressed air.
The present invention does not limit the composition system (kind) of the ferrite to be produced but may be applied to formation of a thin film of a Zn ferrite, a Cu ferrite, a Nixe2x80x94Zn ferrite, a Mnxe2x80x94Zn ferrite and so forth. Therefore, in embodying the present invention, the kind of metal nitrate or metal alkoxide to be contained in the above-mentioned raw material mixed solution may be selected appropriately depending on the composition system. Specifically, in case of producing a thin film of a Zn ferrite, a mixture of Fe(NO3)3 and Zn(NO3)2 is selected as the metal nitrate and a mixture of an Fe alkoxide and a Zn alkoxide is selected as the metal alkoxide. In case of producing a thin film of a Cu ferrite, a mixture of Fe(NO3)3 and Cu(NO3)2 is selected as the metal nitrate and a mixture of an Fe alkoxide and a Cu alkoxide is selected as the metal alkoxide. Further, in case of producing a thin film of a Nixe2x80x94Zn ferrite, a mixture of Fe(NO9)9, Zn(NO3)2 and Ni(NO3)2 is selected as the metal nitrate and a mixture of an Fe alkoxide, a Zn alkoxide and a Ni alkoxide is selected as the metal alkoxide. Still further, in case of producing a thin film of an Mnxe2x80x94Zn ferrite, a mixture of Fe(NO3)3, Zn(NO3)2 and Mn(NO3)2 is selected as the metal nitrate and a mixture of an Fe alkoxide, a Zn alkoxide and a Mn alkoxide is selected as the metal alkoxide.
In the case where a Nixe2x80x94Zn ferrite thin film is to be produced, the metal nitrate or metal alkoxide is preferably compounded such that ion concentrations of (Fe)2, Zn and Ni in the ferrite thin film fall within the following ranges:
0.30xe2x89xa6axe2x89xa60.70, 0.14xe2x89xa6bxe2x89xa60.45, and 0.08xe2x89xa6cxe2x89xa60.38
where a, b and c are ion concentrations of (Fe)2, Zn and Ni, respectively, and a+b+c=1.
Also, in the case where a Mnxe2x80x94Zn ferrite thin film is to be produced, the metal nitrates or metal alkoxide is preferably compounded such that ion concentrations of (Fe)2, Zn and Mn in the ferrite thin film fall within the following ranges:
0.30xe2x89xa6axe2x89xa60.70, 0.15xe2x89xa6bxe2x89xa60.38, and 0.10xe2x89xa6cxe2x89xa60.55
where a, b and c are ion concentrations of (Fe)2, Zn and Mn, respectively, and a+b+c=1.